The present invention relates to processes for the production of concentrated hypochlorite solutions, and more particularly, it relates to continuous processes for the production of concentrated sodium hypochlorite solutions.
According to the present invention, the concentrated sodium hypochlorite solutions are obtained utilizing a two-stage completely continuous chloration which permits the use of chlorine or chlorine diluted with inert gases. The first stage comprises a simple chlorine absorption column without sodium chloride precipitation. The second stage allows a concentrated sodium hypochlorite solution to be obtained after separation of the sodium chloride which is precipitated.
The concentrated sodium hypochlorite solutions contain about 25% NaClO, about 100.degree. chlorometry being obtained in either a continuous or discontinuous operation. The difficulty in obtaining such solutions is the necessity to react a solution of concentrated sodium hydroxide, on the order of about 50% by weight, with dilute or concentrated chlorine. Under these conditions, the sodium chloride which precipitates with a relatively fine crystal size causes a high viscosity of the reaction medium. This salt is then difficult to remove from the solutions and retains, after filtration, considerable quantities of the mother liquor, all of which causes a loss of active chlorine. Moreover, to avoid decomposition of the sodium hypochlorite formed, it is necessary to keep a sufficient excess of sodium hydroxide in the sodium hypochlorite solution.
There exist techniques which permit overcoming these difficulties. French Pat. No. 1,352,198 describes a technique for obtaining 100.degree. chlorometry sodium hypochlorite, which is about 25% NaClO, which provides for solving the crystal size problem of the salt and regulates the excess sodium hydroxide. Nevertheless, this technique is poorly adapted to use residual chlorine more or less diluted by inert gases.
Japanese Pat. Nos. 49-47292/74 and 54-118398/79 of Kokai claim obtaining concentrated hypochlorite solutions containing about 25% NaClO by a discontinuous process in a number of stages. To avoid the introduction of chlorine directly into a 50 weight percent sodium hydroxide and the formation of fine particles of sodium chloride, these two patents operate as follows: In a first stage, a 20% solution of sodium hydroxide is simultaneously introduced with some more or less diluted chlorine. A sodium hypochlorite solution containing about 12% NaClO is obtained. At the end of this first stage in the first patent, a 47% sodium hydroxide and pure chlorine are fed simultaneously up to a chloration amount of 75%. The sodium hydroxide feed is then interrupted and only the chlorine feed is maintained up to a chloration amount of about 98%. In the second of these two Japanese patents, when the total amount of sodium hydroxide is introduced into the solution to a 12% sodium hypochlorite, this causes a precipitation of sodium chloride and diluted chlorine is added to obtain a chloration titer on the order of 98%, where the NaClO content is about 25%.
In these two cases, the processes are necessarily discontinuous because, to obtain sodium chloride having a sufficient crystal size (a mean crystal size of 200 microns is only attained) to assure a good filtration and a small mother liquor retention, it is indispensable in the case of these two patents to introduce the reactants discontinuously.